The present invention relates in general to self-propelled center pivot irrigation systems which are comprised of individual span units, each span unit having a power frame for propelling the span unit and each power frame having drive motor, drive wheels and transmission means which connects the drive motor to the drive wheels and transmits power from the drive motor to the drive wheels. In particular, the present invention relates to the power transmission means.
Heretofore, it has been common in transmissions for center pivot irrigation systems to utilize a double worm gear power transmission between the drive motor and the drive wheel. This transmission was connected either directly to the drive wheel or was connected to a sprocket and drive chain assembly which in turn was connected to the drive wheel.
It has been found in these prior embodiments that worm driven reduction gears are subject to extremely high frictional losses at a low RPM. The use of double worm reduction gearing in the conventional transmission means results in severe power losses due to friction at low RPM's. Further, at low speeds and under heavy load, worm gear assemblies have a tendency to force the lubricant from between the intermeshing gears and tend to run dry. This necessitates the use of excessive lubricant in order to assure adequate lubrication. This is specifically disadvantageous in the rural environment in which the transmission means is specifically applied, as the excess lubricant entraps substantial amounts of dirt, dust and abrasive material which is constantly being stirred up by the action of the drive wheels in the loose dirt in an agriculture field.
Due to the nature of the environment in which the transmission is used, the transmission most frequently operates in the low RPM, high load condition. Self-propelled irrigation systems also generally operate in an on/off mode. The individual drive wheels are continuously stopped and restarted and with each restart the transmission begins operation in a low RPM, high load condition. Specifically, the high frictional losses inherent in a double worm reduction coupled with the loss of lubricant due to the pressure between a worm and a worm gear in a low RPM, high load condition combine to require the use of excessive amounts of drive power. Additionally, the problem of lubricant being forced from between the worm and worm gear tends to produce a potential for running without lubricant and hence shortening the operating life.